xen: Add populate_pt_range interface to reserve non-leaf level table entries

xen: Add populate_pt_range interface to reserve non-leaf level table entries

On x86, for the pages mapped with PAGE_HYPERVISOR attribute
non-leaf page tables are allocated with valid pte entries.
and with MAP_SMALL_PAGES attribute only non-leaf page tables are
allocated with invalid (valid bit set to 0) pte entries.
However on arm this is not the case. On arm for the pages
mapped with PAGE_HYPERVISOR and MAP_SMALL_PAGES both
non-leaf and leaf level page table are allocated with valid bit
in pte entries.

This behaviour in arm makes common vmap code fail to
allocate memory beyond 128MB as described below.

In vm_init, map_pages_to_xen() is called for mapping
vm_bitmap. Initially one page of vm_bitmap is allocated
and mapped using PAGE_HYPERVISOR attribute.
For the rest of vm_bitmap pages, MAP_SMALL_PAGES attribute
is used to map.

In ARM for both PAGE_HYPERVISOR and MAP_SMALL_PAGES, valid bit
is set to 1 in pte entry for these mapping.

In vm_alloc(), map_pages_to_xen() is failing for >128MB because
for this next vm_bitmap page the mapping is already set in vm_init()
with valid bit set in pte entry. So map_pages_to_xen() in
ARM returns error.

With this patch, MAP_SMALL_PAGES is dropped and arch specific
populate_pt_range() api is introduced to populate non-leaf
page table entries for the requested pages. Added RESERVE option
to map non-leaf page table entries.

Signed-off-by: Vijaya Kumar K<Vijaya.Kumar@caviumnetworks.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- rewrote subject line ]